In the area of the conversion of hydraulic energy into mechanical or electric energy, using a hydraulic machine such as a turbine, pump or Francis-type turbo-pump is known. In turbine mode, the hydraulic machine rotatingly drives a shaft thus transforming hydraulic energy into mechanical energy. The conversion of energy is realized by a wheel which forms a rotating part and includes runners which can be distributed around the axis of rotation of the wheel. The runners extend between a first edge and a second edge which are respectively, during operation, a leading edge and a trailing edge for the flow of water.
One of the challenges persisting in a Francis turbine is the research into the best quality of flow possible downstream of the rotating part. For example, it is important to be able to master the distribution of velocities of the flow leaving the wheel, to avoid cavitation phenomena or also to preserve the stability of the flow, that is to say to avoid the formation of turbulences or vortices.
To do this, a wheel with non-movable runners can have an optimum operating flow rate for which the direction of the water leaving the trailing edge of the runners is relatively parallel to the axis of rotation of the wheel with a relatively uniform distribution of output velocity. In such a configuration, the direction of the water leaving the trailing edge can have a limited rotational component, and the nature of the flow allows harmful phenomena such a vortex to be avoided within the intake duct.
On the other hand, for a flow rate that is different from the optimum operating flow rate, the rotation of the wheel causes the water, leaving the trailing edge, to have a direction which is deflected with respect to the axis of rotation of the wheel and at the same time there is deterioration in the uniformity of the output velocities. Thus, the flow leaving the wheel is of a lesser quality and is even more likely to give rise to an increase in losses, vortices and instabilities.
Similar issues arise with other types of turbines, notably for propeller-type turbines.
In order to address this issue, it is known to use controlled mechanisms to displace the runners of the Kaplan, Bulbe or Deriaz turbines according to the turbine flow rate so as to maintain a good quality of flow downstream of the wheel. However, such mechanisms increase the end cost of the turbine and their use is limited to restricted heights of head applied to turbines.